The present invention is directed, in general, to a method of controlling molecular weight distributions during a polymerization process and, more specifically, to a method of controlling molecular weight distribution during a polymerization process in an up-flow heat exchanger and in the presence of an initiator.
The polymerization of styrene is a very important industrial process that supplies materials used to create a wide variety of polystyrene containing articles, such as cups, thin films and foams. The use of polystyrene in this wide range of articles results from the ability to fine tune the parameters of the polymerization process. Thus, these variations in the polymerization conditions are of utmost importance since they allow control over the physical properties of the resulting polymer. These physical properties determine the suitability of a polystyrene for particular applications. Properties of particular importance are the weight averaged molecular weight (MW) of the polymer, molecular weight distribution (MWD), and melt flow index (MFI). For a desired application, these physical characteristics must be controlled and optimized to achieve a polystyrene material that will have characteristics within the tolerances of desired product.
One general method of controlling these properties requires processes by which the low molecular weight components, such as starting monomer, dimers and trimers, are removed from the desired polymer product. Vacuum distillation is one method for removing these components. As the name implies, vacuum distillation subjects the product mixture to low-pressures to extract the volatile components. Another method known as flash volatilization may involve the application of low pressure as well as heat to further extract remaining volatile components. In addition to these methods, chemical agents, such as steam, can be used to strip volatile components from the polymer product stream. Processes which employ one or more of these approaches are known in the art.
U.S. Pat. No. 3,311,676 issued to Toekes teaches a method for removing low molecular weight components of a polystyrene by using a preheater, a heat-exchanger, and a low-pressure phase separator. In Toekes the pre-heater heats the reaction mixture and the heat exchanger maintains this temperature at the reduced pressure generated by the phase separator. This method produces a foam that allows for rapid removal of volatiles and results in a monomer and ethylbenzene concentration below about 1000 ppm.
U.S. Pat. No. 3,865,672 issued to Mertzinger disclosed a process for removing volatiles from a polymer using a single stage vacuum distillation system. More importantly, the process employs a vertical heat exchanger operated in a down-flow configuration. In a down-flow configuration the polymer mixture is fed to the top of the vertical heat exchanger and volatile components are removed as the mixture flows downward toward the end of the heat exchanger. In this system the mixture is subjected to greater temperatures as it flows toward the bottom of the down-flow heat exchanger. Mertzinger reports that the concentration of volatiles achieved by this process is reduced about 3000 ppm.
A down-flow falling strand devolatilizer is disclosed in U.S. Pat. No. 3,928,300 to Hagberg. In this process heated polymer is extruded through a plurality of apertures at the top of a reduced-pressure vessel. The extrusion of the polymer through the apertures increases the surface area of the mixture, thereby facilitating the removal of volatile components.
U.S. Pat. No. 5,540,813 issued to Sosa, et. al. (Sosa ""813), which is incorporated herein by reference, discloses a method and apparatus method that reduces residual monomer content by a combination of serially arranged heat exchangers and devolatilizers in conjunction with the polymerization reactor system. Upon exiting the reactor system, the polymer mixture enters an up-flow type heat exchanger before entering a down-flow heat exchanger under reduced pressure. The mixture then enters a second devolatilizer containing a hoop nozzle manifold. By controlling the temperature in the various modules, this method not only allows removal of substantial portions of volatile components, but also allows strict control over parameters such as the molecular weight distribution and the melt flow index.
However, as production temperatures approach their upper limit, increasing reaction and devolatilization temperatures to achieve production improvements become very problematic. Therefore, what is needed in the art is a process by which substantial portions of volatile components can be eliminated while maintaining strict control over molecular weight, molecular weight distribution and melt flow index, yet does not require higher temperatures nor additional process steps.
To overcome the deficiencies of the prior art, the present invention provides a method of producing a polymer. In a preferred embodiment, the method includes taking a product stream from a monovinyl aromatic polymerization system wherein the product stream comprises a polymerized monomer and a monomer. The product stream is introduced into an up-flow heat exchanger in a presence of an initiator, such as a peroxide, that affects a polymerization of the monomer in the up-flow heat exchanger. In one advantageous embodiment, the initiator includes a first initiator and a second initiator where a ratio of the second initiator to the first initiator can range from about 0:600 to about 50:600. In a more specific embodiment, the ratio of the second initiator to the first initiator in the portion ranges from about 0:400 to about 50:400 and a ratio of the second initiator to the first initiator in the remaining portion is about 0:200.
In a particular embodiment, the first initiator may be a low-temperature initiator and the second initiator may be a high-temperature initiator. Alternatively, the second initiator may be an intermediate-temperature initiator.
In another advantageous embodiment, the method further includes forming a polymerized monomer in the up-flow heat exchanger to form a second product stream and devolatilizing the second product stream to form a polymerized monomer having a molecular weight distribution ranging from about 2.8 to about 3.3.
In yet another embodiment, the present invention provides a method of producing a polystyrene which includes taking a product stream from a styrene polymerization system that includes polystyrene and styrene and introducing the product stream into an up-flow heat exchanger in a presence of an initiator, which affects a polymerization of styrene.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.